The present invention relates generally to electronics, and more specifically relates to a circuit for indicating a charge state of a battery pack used with battery power driven devices.
There has been continued innovation in the field of battery packs, and especially rechargeable battery packs that are used with portable devices, such as cordless power tools. Examples of such rechargeable battery packs are those produced under the SKIL and BOSCH brands by the S-B Power Tool Corporation of Chicago, Ill., which are used with various cordless power tools also produced under the same brands. A typical rechargeable battery pack ordinarily has a generally rectangular housing and a male stem connector extending from a surface thereof. These battery packs commonly have an associated battery charger that is correspondingly sized and configured to contain a female slot for receiving the male stem connector of the battery pack. The battery packs are further equipped with terminals for establishing electrical contact with associated mating terminals of the battery charger, or alternatively battery power driven devices.
For many devices using rechargeable battery packs, such devices are provided as a kit and often include, in addition to the device itself, two battery packs and a single battery charger. Having multiple battery packs of a given type enables the user to continuously use the cordless power tool by swapping out depleted battery packs for filly charged battery packs. A common problem for users of such kits, however, is determining the charge states of the battery packs. In some instances, operators may have not used the kits in some time, and thus are unsure of the charge state of the battery packs and may charge filly charged battery packs. At other times, unless the battery packs are discharged resulting in the device being inoperable, operators may be unsure of when battery packs need to be recharged. This can result in the battery packs being charged unnecessarily.
For many types of rechargeable battery packs, such as those using Nickel-Cadmium (NiCd) batteries, it is desirable to deplete an existing charge of the battery pack prior to its recharging to extend the battery pack's lifetime. Moreover, it is also desirable to extend a battery pack's lifetime by limiting the number of times the battery pack is recharged. Heavy users of such battery packs would benefit from an indicator of a charge state of the battery pack. Such users would be able to perform optimal charging of the battery packs and minimize excessive charging by adjusting their charging time according to an indicated charge state.